Story

The story begins with a human scientist named Ciel and her reploid companions being chased through an underground bunker. Their relentless pursuers are mass produced X look-a-likes called Pantheons, among other terrible machines. After heavy losses, Ciel and company arrive at a sealed chamber containing Zero who has been powered down for 100 years.

After sleeping for a hundred years, Zero awakens to a world where reploids are being constantly accused of being Mavericks and systematically "retired", apparently under the direction of X himself. Assisting Ciel in her escape from the bunker, Zero begins his fight against X's utopia, Neo Arcadia, and its Four Guardians: Harpuia, Leviathan, Fefnir, and Phantom.